Power generation continues to be an important application of rotating electrical machines. Wind energy is one of the fastest growing sources of electricity in the United States and around the world, and wind turbines employing rotating electrical machines are used to convert wind energy to usable power. A conventional wind turbine includes a turbine rotor having turbine blades and output shaft which drive an electrical machine that can supply 3-5 Megawatts of power to the utility power network. Due to the relatively slow rotation of the turbine blades and shaft (up to 15 revolutions per minute), the generator component of a conventional wind turbine includes, along with the electrical generator and control electronics, a gearbox for converting the low speed incoming rotation to high speed rotation suitable for generating electricity. In addition, to protect the turbine blades, output shaft and bearings from instantaneous over-torque due, for example, to pulses from a terminal fault, the turbine output shaft and gearbox may include protection devices such as slip planes as an integral part of the gearbox or as a coupling between the generator rotor and the gearbox.
Increased power demand is leading to increased power requirements for each wind turbine. To obtain a wind turbine that can deliver 8-10 Megawatts, a mere scaling up of the size of the conventional power train becomes impractical, due at least in part to the size, weight and cost of a gearbox which can accommodate these requirements.